First time grow/er Thoughts please

Hi all. I have been researching for months now and have finally got my grow cab up and running w/ 2 small seedlings unknown type started.
Setup:
-Dimensions: 4'W X 2'D X 6'T
-400w MH/HPS W/6" cooltube
-Carbon filter mounted in cab on inlet of light
-6" (400cfm) inline fan in attic(exhaust)
-Soil(happy frog and ff mix) 1/2&1/2

I got these (hopefully)girls 5 days ago in 1Gal pots. I transplanted them into 3Gal pots yesterday. I have not fed them any nutes yet and have lightly watered them twice. First time with tap water then when transplanted with bottled water just to be safe. 1L each pot. I noticed some yellowing around one of the plants leaves a few days ago and seems to be getting worse. Does it look like a nute deficiency? I have seen many pics of other sick plants but nothing exactly. I have the fox farm trio for nutes. Should I feed them (w/ 1/4 of the recommended)and see if that helps. I have checked for bugs w/ magnifying glass and have found none. Just wondering what path of troubleshooting to should gther than that how do they look? Thanks

that yellowing looks to be more of a light burn than a nute deficit to me m8.. how close are the lights to the plants/?? i know i will burn some of mine once in awhile when they grow a bunch over night and get into the lights and that's what mine tend to look like when it happens..

how old are they now?? i usually wait till about 3 or 4 weeks from seed to start feeding them, or if they look hungry before that.. but yah, when you start to feed them, start with 1/4 of w/e the bottles say, and work your way up from there..
other then the burnt bits on the one, they look good imo..

They are 3wks old since they popped from seed. The light is about 18" away from plant. I will give raising it a little a shot. Thanks for the quick response. I really appreciate it.

it looks like light burn but that one top leaf that it started on seems like its still green just mutated (might be my perspective). if it is healthy in one spot then goes white its light burn, had this happen when i went from my 400 to my 1000 but only happened on my trainwreck. but if your using cfls u shouldnt have to raise but keep an eye out have a growtastic day

Yo mang, you need to add some fertilizer.
Roots like phosphorus, give them some....now.
I used Organicare Grow and Bloom dry fertilizer once. I would add it to a 5 gallon jug of tap water, and aeriate it for a few days with some Great White, a few tablespoons of earthworm castings, Sucanat, epsom salt, and a few drops of lemon juice to pull the ph down a little, and I had no problems at all and great nugs.
But if you already got some, then I agree with your idea of feeding at 1/4 strength